Question: Simplify the following expression: $9\sqrt{20}$.
First, try to factor a perfect square out of the radical. $= 9\sqrt{20}$ $= 9\sqrt{4 \cdot 5}$ Separate the radical and simplify. $= 9\sqrt{4} \cdot \sqrt{5}$ $= 9 \cdot 2 \cdot \sqrt{5}$ $= 18\sqrt{5}$